The invention relates to land-based or satellite-based radio telephone systems using frequency hopping and methods to reduce interference between cells using the same frequencies at the same time.
In British Patent No. 8118954, and corresponding U.S. Pat. No. 4,476,566 to Applicant entitled "Frequency Hopping Radio Communications Apparatus Having Pseudo-Random Channel Selection", an orthogonal frequency hopping channel selection method is described that permits a radio station within a given group to select, for successive transmission intervals, a random channel to use for communication. At the same time, this method provides that different stations of the same group select different channels during the same interval and thus do not interfere with one another. The above patents are hereby incorporated by reference herein. This method of channel selection is termed "Orthogonal Frequency Hopping" as opposed to "Random Frequency Hopping" in which each station selects a channel at random and thus has a random chance of selecting the same channel as another station. These systems make it difficult, if not impossible, to deduce the particular channel that a second station will select based on the channel selected by a first station, except for the fact that it will be different. This feature of conventional orthogonal frequency selection teachings was useful in military applications in which it was desirable to hinder deliberate jamming by an enemy.
One application of the current invention is a civilian application such as cellular radio telephony. In cellular radio telephony, service of as many mobile telephone users as possible with a limited number of radio channels is desirable. The available radio channels are re-used many times over across a continent to achieve the required capacity, but conventional techniques assign channels to geographical cells such that adjacent cells do not use the same channel, thus avoiding excessive interference. The concept of frequency re-use involves dividing the total number of frequency channels into a number M of subgroups, and allocating each of the m subgroups for use in a cell according to an M-cell reuse pattern, such that cells allocated the same subgroup are separated by root(M) cell diameters between cell centers. The larger M is selected to be, the greater the distance between cochannel interfering stations; however fewer channels are available for use in each cell. Thus interference is reduced at the expense of capacity, so it is desirable to discover other ways of rendering interference more tolerable than by increasing the size M of the re-use pattern.
Conventional fixed frequency re-use patterns have the characteristic that a given station in one cell always suffers cochannel interference from another given station in another cell that is assigned the same channel. The interfering station may sometimes be a station near the station with which it is in communication (thus using a low transmission power), but on other occasions it may be a station using maximum power. Since the interference situation prevails for the duration of a cellular telephone call, it is necessary to plan to be able to cope with worst case situations, and so conventional fixed frequency reuse plans tend to be over-conservative.
Another method that provides an improvement over fixed frequency re-use plans is the frequency hopping method specified for the European cellular system known as the Global System for Mobile communications (GSM). In GSM, the subgroup of frequencies assigned for use within a particular cell is not divided to provide a single, unique frequency for each station in the cell, but rather each station is programmed to select a frequency at random from the entire subgroup according to an orthogonal frequency selection process. The stations within a first cell are thus "orthogonal" to one another, and have no frequency overlap with adjacent cells using different frequency subgroups. In a second cell that is assigned the same group of frequencies as the first cell, the stations are again orthogonal to one another but are conditioned to perform orthogonal frequency selection according to a pseudo-random selection algorithm different from that of the first cell, such that a station in the first cell is not always interfered by the same station in the second cell, but one of the stations in the second cell selected at random from one interval to the next. When selecting a station at random there is a 50% probability of its transmission being silent due to voice or data traffic activity factor, therefore the incidence of frequency clashes is reduced on average by 50% and the probability of a clash is uncorrelated from one interval to the next. Moreover, the power level of a clash varies depending on whether the interfering station is using high or low transmit power. By interleaving error-correction coded data frames over several consecutive frequency hop intervals, data can still be decoded satisfactorily with a random percentage of hops interfered more strongly than average. Thus GSM's frequency hopping provides interferer averaging, allowing the reuse pattern size M to be reduced compared to the value of M that would be needed to survive worst case interference. This translates to an increase in the number of calls that can be handled, i.e. capacity.
An improvement to frequency hopping of the GSM type is disclosed in U.S. Pat. No. 5,425,049 to Applicant, entitled "Staggered Frequency Hopping Cellular Radio System". This patent, which is hereby incorporated by reference herein, discloses an advantage in deliberately offsetting the timing of frequency hopping intervals between cells that are assigned to hop over the same subgroup of channels. When the timing offset is a fraction, such as 1/3rd of the hop interval, clashes with different interfering stations occur in each 1/3rd of the hop interval, thus providing even more interferer averaging.
Another benefit of frequency hopping is that it can average out frequency selective fading. If, due to multipath propagation, destructive interference occurs on some frequency channels, that situation will only occur at random on certain frequency hops and the outage event can be handled by the interleaving and error correction coding. To obtain the maximum gain against frequency selective fading, it is desirable to frequency hop over as many channels as possible. However, the number of channels used by any one cell were, in the aforedescribed conventional frequency hopping scheme, still a factor M less than the total number of channels available, with M on the order of 3 to 9. When practicing the current invention described below however, all stations may frequency hop over the entire number of channels available, thus achieving the maximum advantage against frequency selective fading.
In another application, a cellular radio telephone system is created with the aid of orbiting satellites equipped with multi-beam antennas, each antenna beam being associated with a geographical cell or service area on the ground. The geographical regions associated with a given satellite beam may be fixed, as when geostationary satellites or used, or moving satellite equipped with electronically steered beams; alternatively the geographical regions served by a particular satellite beam may be moving with the motion of the satellite in orbit round the earth. Nevertheless, within the appropriate moving or fixed frame of reference in which the beams are static, the need for inter-beam frequency reuse strategies can arise in order to control beam-to-beam interference. These matters are discussed in the following U.S. patents to Applicant, which are hereby incorporated by reference herein:
U.S. Pat. No. 5,642,358 entitled "Multiple beam width phased array";
U.S. Pat. No. 5,631,898 entitled "Cellular/satellite communications system with improved frequency re-use";
U.S. Pat. No. 5,619,503 entitled "Cellular/satellite communications system with improved frequency re-use";
U.S. Pat. No. 5,619,210 entitled "Large phased-array communications satellite";
U.S. Pat. No. 5,594,941 entitled "A cellular/satellite communications system with generation of a plurality of sets of intersecting antenna beams";
U.S. Pat. No. 5,566,168 entitled "TDMA/FDMA/CDMA hybrid radio access methods";
U.S. Pat. No. 5,555,257 entitled "Cellular/satellite communications system with improved frequency re-use"; and
U.S. Pat. No. 5,539,730 entitled "TDMA/FDMA/CDMA hybrid radio access methods".
In one frequency band allocated for satellite communication to mobile phones, usage of the band is conditioned upon the average energy in any 4 KHz part of the frequency band reaching any square meter of the earth being below a specified limit. Thus narrowband communications systems that concentrate energy into a few, narrow channels would exceed the specified spectral density limit. Higher energy may be transmitted by using a spread-spectrum form of transmission such as Direct Sequence Spread Spectrum Multiple Access (DSSSMA), also known as Code Division Multiple Access (CDMA), or alternatively Frequency Hopping Spread Spectrum (FHSS). When FHSS is used, the frequencies used in any area, i.e. in that region of the earth served by a particular directive beam, should be hopped over the entire number of channels available in order to spread the energy density as thinly as possible, and not hopped over a subset of channels only. Nevertheless communications in adjacent beams should preferably not use the same channels at the same time. The need arises therefore for a frequency hopping system in which frequency selection is orthogonal within a cell and its adjacent cells, while in non-adjacent cells the probability of a frequency clash between any two given stations is preferably random and not an event of long duration. These requirements and improvements are achieved when practicing the inventive orthogonal frequency hopping system described herein.